This invention relates to logging apparatus used in boreholes, and more particularly to an improved sonde housing for such apparatus.
Logging apparatus which is used for the study of formations penetrated by a borehole includes a sonde lowered into the borehole at the end of a cable. The sonde is generally made up of several sealed housings connected end to end mechanically and electrically. These sealed housings can be filled with air at atmospheric pressure or with a hydraulic fluid kept at the pressure of the borehole. They contain transducers for the acquisition of measurements, and electronic circuits for the electric power supply of the transducers or processing of detected measurement signals. These housings are generally metallic cylinders equipped with heads mounted in a sealed manner at their ends.
For certain logs, it is not possible to use housings made of conductive material, as this would constitute a screen between the transducers and the formations. Such logs particularly include those involving the use of coil or antenna type transducers for the measurement of electrical resistivity (or conductivity), dielectric constant, nuclear magnetic resonance, or other characteristics of the formations traversed by the borehole.
U.S. Pat. No. 2,857,451 (H. A. Barclay) describes a nonconducting housing capable of resisting borehole pressures. That housing is made up of internal ceramic cylinders surrounded by an external cylinder made up of a fabric impregnated with thermosetting plastic material. The internal cylinders resist pressure, while the external cylinder provides sealing and is resistant to axial tensions and shocks.
Another nonconducting and pressure-resisting housing is described in U.S. Pat. No. 3,403,328 (F. N. Fossati et al) and a particular embodiment is represented in FIGS. 7A and 7B thereof. That housing includes a stack of internal ceramic discs and an external layer of glass-fiber reinforced resin. In addition, glass-fiber strips are tensioned between the ends of the sonde to apply an axial-compression prestress to the stack of discs. The discs withstand the borehole pressure and the intermediate layer resists longitudinal tensions.
In the above-described housings, thought was given to the use of stacked ceramic rings whose compression strength is very high. The low tensile strength of the ceramic material is compensated by an additional layer adapted to support the longitudinal forces. Those housings however have several drawbacks. Ceramic material is very sensitive to shocks. Since the rings are in contact with each other, the shocks are transmitted directly from one ring to another with no damping. One or several rings are thus capable of breaking, in particular during surface operations for introducing the apparatus into the borehole, during which substantial shocks can occur. Further, under the severe conditions prevailing in boreholes, the resins are subjected to more or less rapid deterioration due to hydrolysis by the borehole fluids. Such degradation is particularly troublesome at the junctions between the resin and the connecting parts of the ends, where the traction forces are applied.
It is therefore an object of the invention to provide a nonconducting housing for logging apparatus which withstands the borehole pressure and resists shocks in a particularly satisfactory manner.
Another object of the invention is to provide a nonconducting, pressure-resisting housing exhibiting excellent resistance to pressure and degradation by borehole fluids.
The object of the invention also covers a method for manufacturing such a housing.